TRAVERSE CITY -- Video images from the bottom of the St. Clair River have turned up no evidence that erosion caused by dredging is causing water levels on the upper Great Lakes to drop, scientists said Thursday.

But they emphasized their research is in the early stages and said no final conclusions have been reached.

The International Joint Commission, a U.S.-Canadian agency that advises both nations on Great Lakes issues, is overseeing a study of whether erosion near the upper end of the St. Clair River is a cause of the steady decline in the levels of Lakes Michigan and Huron since the late 1990s.

The Georgian Bay Association, a Canadian lakefront homeowners group, has released two reports contending human activities such as 1960s dredging opened a "drain hole" in the river bed, causing the two lakes to lose up to 2.5 billion gallons a day.

The IJC study began six months ago, with final results scheduled for release in 2009.

In a progress report released Thursday in Toronto, study leaders said video footage taken earlier this year showed the deepest section of the river bed was "fully armored," meaning covered with a stable layer of rocks and pebbles.

"Therefore, the bed cannot be eroding," the report said.

But it remains unclear whether the rock layer was uncovered by previous erosion, so it's too early to say whether the Georgian Bay Association's theory is correct, said Ted Yuzyk, co-chairman of the IJC study group. Other measurements will be taken to provide further information, he said.

"On a preliminary basis, we're finding that ongoing erosion does not appear to be a cause of low water levels," IJC spokesman John Nevin said.